Ammunition rounds from which projectiles are propelled by pressurized gases, are well known in the art, together with associated igniters, explosives, propellant charges and other common means as disclosed for example in U.S. Pat. Nos. 4,770,099, 4,799,428 and 5,123,356 to Brede et al., Yunan and Brooks et al., respectively. Usually in such prior art ammunition rounds the burning propellant is the sole gas generating source for build up of pressure within the gun chamber until motion resisting friction and inertia are overcome and the projectile starts moving. As the projectile moves forward, the accompanying increase in chamber volume relieves some of the rapid buildup of pressure due to gas generation. The peak value of the pressure is determined by competition between rate of gas generation and rate of volumetric expansion. Because the rate of gas generation depends on the amount of propellant, the strength of the gun limits the acceptable amount of propellant that can be safely burnt, which in turn limits the muzzle velocity of the projectile.
It is therefore an important object of the present invention to provide an ammunition round in which projectile movement begins in response to an independent source of gas pressure so as to safely accommodate a larger quantity of propellant usually resulting in increased muzzle velocity of the projectile.